Magnetic resonance imaging has developed into a standard method in medical diagnosis since the invention of rapid imaging. A plurality of recording methods exists, which differ inter alia in terms of the required measuring time and the resulting image contrasts. Methods for recording so-called spin density or proton density-weighted, T1-, T2- or T2*-weighted images exist, furthermore, diffusion imaging is also known.
If the term recording of images, is used below this naturally means that, as conventional in magnetic resonance tomography, a slice of the examination object located in the magnetic resonance tomography system is excited by way of an excitation pulse and a slice selection gradient, the signal of the excited spin is read out with a detection coil and is transmitted via a number of electronic processing steps, for instance an AD converter, to a storage facility, and the stored raw data is again converted into one or a number of image data records or images by means of a number of post processing steps, also known as post processing. However it is in line with conventional linguistic usage to refer to the recording of images, as a result of which this wording is also used in the present application.
Other image recording techniques, such as for instance non-selective 3D imaging, which operates without slice selection gradient, can naturally also be used.
The raw data is usually recorded in rows in the so-called k-space, thereby resulting in the following:
As already described, at least one detection coil is used to record the signal. This coil is only able to forward the induced voltage. Therefore only a temporally varying voltage signal is obtained. In order to be able to obtain an item of position information from a signal of this type, magnetic field gradients are used, in order to vary the resonance frequency of the signal in a position-dependent manner and thus to obtain an item of position information. For position encoding in a spatial direction, a magnetic field gradient, the so-called read-out or read gradient, can be connected during the reading out of the signal. In order to encode the signal in further spatial directions, so-called phase gradients are used. The signals which are recorded with a specific value of the phase gradient form a line in the k-space. Depending on the resolution in the spatial direction encoded by the phase gradients, the phase gradient is connected at different strengths. The line at which the phase gradient has the value 0 forms the center of the k-space.
Two phase encoding gradients or with 3D recording methods also three phase gradients can however also be used. The use of more complex recording schemes, such as for instance spiral or radial imaging with special k-space trajectories, is likewise also possible. The center of the k-space is the respective point at which all phase gradients have their minimal value.
Since, apart from the value of the phase gradient, the sequence of HF pulses and magnetic field gradients always repeats at least in the basic sequences, these can be shown in a sequence diagram, in which only one cycle between the excitation of the spin and the read-out of the signal and if necessary preparation modules used are shown.
The gradient echo sequence (GE), the spin echo sequence (SE), the turbo spin echo sequence (TSE) and also the true fisp sequence are inter alia regarded as basic sequences.
Imaging sequences derived herefrom are for instance the Short Inversion Recovery (STIR) sequence or the Fluid Attended Inversion Recovery (FLAIR) sequence. These are based on the SE sequence in that prior to the 90° excitation pulse of the SE sequence, a 180° pulse, also known as inversion pulse, is switched. The time between the 180° inversion pulse and the 90° excitation pulse is the inversion time TI. The difference between the STIR sequence and the FLAIR sequence only lies in the inversion time TI, which lies in the range of hundreds of milliseconds in the case of the STIR sequence and at two seconds in the case of the FLAIR sequence. Furthermore, a plurality of further variations in standard sequences exist, for instance for flux compensation.
On account of the plurality of possibilities, it is not possible, with the aid of individual setting values such as echo time TE or repetition time TR, to predict the resulting contrast behavior of the magnetic resonance image. Contrast behavior is understood to mean the signal intensity difference of the various tissues imaged in the magnetic resonance image, caused by the different relaxation behaviors and proton densities. The contrast behavior is often also abbreviated to contrast, but the term “contrast” may basically have an additional connotation. In the present application, the contrast behavior is meant with the use of the term ‘contrast’.
An assignment or knowledge of the contrast behavior of magnetic resonance images is however desirable in order automatically to display to a user, using a display facility, in an optimized arrangement adjusted in particular to a medical question, the images with different contrasts.